A new nonstationary model is applied to represent seismic ground acceleration records. The model can be obtained by modulating both the amplitude and the frequency of a stationary process. It is not included in the class of oscillatory processes. Accelerograms recorded during the 1985 Michoacan earthquake in Mexico City are used to calibrate and evaluate the usefulness of the proposed and other nonstationary models of ground acceleration. Results suggest that the proposed nonstationary model is more adequate for reliability studies. System responses to accelerograms generated by this model have coefficients of variation that are consistent with those corresponding to actual accelerograms.